The US trial - the Diabetes Prevention Program (DPP) clinical trial - is the first to demonstrate very long-term effects of any medication on weight loss. The trial followed the participants for 15 years.

A total of 3,234 obese or overweight individuals with prediabetes were recruited. Some volunteers took metformin, another group were given a placebo, and the remaining participants were required to follow a lifestyle intervention.

After one year, 28.5% of the metformin group had lost at least 5% of their weight. In comparison only 13.4% in the placebo group reached this target.

Predictably, those in the diet and lifestyle advice group were far more successful, with 62.6% losing at least 5% of their weight within the first year.

Interestingly, the metformin group had greater success at keeping the weight off over the next 14 years. A total of 56.4% achieved 5% weight loss after 15 years in the metformin group, compared with 48.9% in the lifestyle group and 41.7% in the placebo group.

Speaking to Medscape Medical News, senior author Dr Kishore M. Gadde, from the Pennington Biomedical Research Center in Louisiana, said: "Those who lost weight initially after one year had remarkable maintenance of the weight loss with metformin, something we had never known before."

At the moment, metformin has only been approved by official prescription drug bodies, in the UK and US, for the treatment of type 2 diabetes. But the research team say their findings indicate that the drug could be considered for those who struggle to keep their weight down in a bid to avoid developing type 2 diabetes.